In general, auxiliary brakes for use in vehicles include a fluid-type retarding device and an eddy-current retarding device.
As disclosed, for example, in Patent Documents 1 and 2, the fluid-type retarding device includes paired impellers as used in a fluid coupling, which are disposed so as to face each other within a working container connected to the rear end of a transmission.
The paired impellers disclosed in Patent Documents 1 and 2 each include radially extending vanes, one of which is a fixed impeller fixed to the working container and serving as a stator, and the other one of which is a rotating impeller serving as a rotor and fixed directly with the output shaft (rotating shaft) of the transmission or fixed through a speed-increasing gear mechanism. The fixed impeller and the rotating impeller form a torus-shaped working chamber.
In the configuration described above, at the time of braking, the working chamber is supplied and filled with working fluid (oil, water, or mixture thereof) through oil-hydraulic pressure or air pressure. Then, the rotating impeller is rotating, while the fixed impeller is at rest. Thus, a difference in relative rotational speed takes place between these impellers, and the working fluid circulates between the rotating impeller and the fixed impeller within the working chamber, thereby causing circulatory flow of the working fluid. At this time, the working fluid functions as a resistance that prevents the rotating impeller from rotating. This leads to a generation of braking force acting on the rotating impeller, whereby it is possible to reduce the speed of rotation of the rotating shaft through the rotating impeller.
At this time, kinetic energy of the rotating shaft is converted into thermal energy in association with the generation of braking force, whereby temperatures of the working fluid increase to be a high temperature. Thus, it is necessary for the fluid-type retarding device disclosed in Patent Document 1 to have an external cooling system that discharges the working fluid, which has high temperatures, into the outside of the working container, thereby cooling it with a heat exchanger.
In the case of this retarding device, although high braking force can be stably generated for a long period of time, a device for supplying and discharging the working fluid as well as the heat exchanger are required in addition to the paired impellers that directly contribute to the generation of braking force, so that the device configuration is complicated, and the weight of the device itself increases. Furthermore, even if it is possible to use the cooling water system that the vehicle originally has, significant modifications are necessary for the vehicle. Thus, this retarding device is not suitable for small to mid-sized vehicles, which have relatively reduced vehicle weight and for which compact and simple assembly is desired.
On the other hand, a fluid-type retarding device disclosed in Patent Document 2 employs an internal cooling system in which a ring-shaped heat exchanger is provided radially outside of the working chamber, and a fan driven by electricity or the pressure of the working fluid is attached to a rotating shaft. With this internal cooling system, the working fluid, which has high temperatures, is guided to the ring-shaped heat exchanger, and the working fluid is air cooled by air blow from the fan. In the case of this retarding device, the external heat exchanger is not necessary, and pipes for connecting with this external heat exchanger can be omitted. Furthermore, this retarding device is separated from the cooling water system of the vehicle, thereby exhibiting excellent assembly properties.
However, as with Patent Document 1, with the fluid-type retarding device disclosed in Patent Document 2, it is necessary to discharge the working fluid from the working chamber during non-braking periods, and supply the working fluid to the working chamber at the time of braking. Thus, it is essential to provide a special mechanism such as a pneumatic system, a hydraulic pump, and a shutoff valve to supply and discharge the working fluid to and from the working chamber, and furthermore, a reservoir container for storing the working fluid is essential. This is a large obstacle to use in small to mid-sized vehicles for which further reductions in the number of parts, weight, and size are desired. Furthermore, it is necessary to supply or discharge the working fluid to or from the working chamber at the time of switching from braking to non-braking and vice versa, and hence, it takes certain time from the non-braking state to obtain a desired braking force or from the braking state to fully become the non-braking state, which inconveniently results in a delay in response.
On the other hand, the eddy-current retarding device includes a brake member connected to a rotating shaft, and causes eddy current on a surface of the brake member facing magnets due to an effect of a magnetic field from permanent magnets or electromagnet at the time of braking. With the eddy current, there occurs braking force in a direction opposite to the rotational direction of the brake member rotating integrally with the rotating shaft, thereby reducing the speed of rotation of the rotating shaft (see, for example, Patent Documents 3 to 6).
In the case of the eddy-current retarding device, with the eddy current generated on the brake member at the time of braking, the kinetic energy of the rotating shaft is converted into thermal energy. With this thermal energy, the brake member is heated, and the heat generated is radiated through fins provided to the brake member, which rotates at high speed.
For these reasons, unlike the fluid-type retarding device, it is not necessary to discharge the working fluid from the working chamber to cool it with the heat exchanger, and the device configuration is simple. In particular, in the case of an eddy-current retarding device employing strong permanent magnets, the permanent magnets have significantly reduced size and reduced weight as compared with the electromagnet that generates the same magnetic force, and hence, it is possible to reduce the weight and the size. Thus, this eddy-current retarding device is promising since it can be applied not only to large-sized vehicles but also to small to mid-sized vehicles.